1. Field of the Invention
This invention relates to an in vitro method of analysing the thrombotic and thrombolytic activity of blood. In particular it relates to a method and apparatus for rapidly and simply assessing the capacity for shear-induced platelet aggregation in native blood.
2. Description of the Related Art
The degree to which platelet aggregation occurs in a sample of blood taken from a patient is an indicator of the likelihood of the risk to the patient of, at one extreme, haemorrhage and the other, thrombosis. An example of the former would be the bleeding disorder of Willebrand disease and. haemophilia which results from a low level of Factor VIII, a blood clotting protein, An example of the latter.,would be advanced artherosclerosis where hyper-reactivity of platelets has been documented. Patients with increased fibrinolytic potential, such as after coronary artery bypass surgery are at risk of excessive bleeding, while decreased fibrinolytic potential i.e. reduced ability to spontaneously dissolve a thrombus may lead to lasting occlusion and myocardial infarction.
There are many known techniques for measuring fibrinolysis. Generally however fibrinolysis assays such as the lysis time of plasma proteins,clot made from whole plasma or whole blood are laborious and time-consuming and therefore rarely used in clinical practice. Fibrinolytic status is normally assessed by the factorial approach wherein at least half a dozen plasma variables are measured. The list of necessary variables is constantly increasing and the assessment of the overall status from the individual variables is extremely difficult.
There are also known techniques for measuring haemostasis i.e. the process of blood protein interaction and cellular activation which leads to the formation of a platelet plug and subsequently to the production of a fibrin-platelet clot at the sight of a tissue injury whereby bleeding is halted and vascular integrity restored. For example European Patent Application No. 0129425 describes a laboratory technique for measuring haemostasis. A polyethylene tube connected to a syringe of fresh blood is punctured with a small hole to simulate bleeding. The bleeding through the hole and clotting time in the tube are monitored. With this arrangement human blood samples can be monitored and evaluated with and without the presence of various agents which promote haemostasis or thrombolytic activity. In a development of this technique as described in PCT/GB87/00633, multiple channels of tubing are connected to individual syringes of freshly drawn blood, which channels are simultaneously punctured by a common punching needle. This permits the concurrent measurement of haemostasis and the expulsion of the haemostatic plug i.e. thrombolysis provided that coagulation of the blood is prevented by an anticoagulant.
In another method described in U.S. Pat. No. 4,780,418, blood to be tested is drawn into a capillary tube under a given reference pressure which is produced with a sophisticated control feedback whereby the amount of blood flowing in the capillary is measured as an indication of the aggregation of the blood platelets. Later workers have sought to ensure that thrombus formation takes place at a precisely defined location in the measuring apparatus by providing an aperture through which blood is passed from a capillary tube, the aperture being formed in a porous member which is generally soaked in a solution which activates the platelets.
In the Applicant's own European Patent Application No. 9390027.6, there is described a method for measuring thrombotic and thrombolytic activity of whole blood in which again blood is drawn under pressure through a capillary tube. However, conditions are arranged such that thrombus formation occurs in the capillary tube solely by shear stress. Once the tube has been occluded by the formed thrombus, the pressure is reduced and the thrombus allowed to stabilise for a period of time. The pressure is then reversed and lysis i.e. dislodgement of the thrombus is monitored by detecting restoration of flow through the tube.
The method disclosed in European Patent Application No. 93900279.6 has been found to provide accurate results relevant to the in vivo situation. However the method like all other physiologically relevant techniques known to the Applicant suffers from the disadvantage that it is time consuming. In particular, the thrombolysis measurement takes a considerable time, up to 90 min, and, until it occurs, the instrument cannot be used for testing another blood sample. Known techniques do not allow for rapid and ready analysis and so are not suitable for use in clinics and the like where many patients are to be tested in a day.
There therefore exists a need for a simple device which allows the evaluation of the test result immediately and is suitable for testing the population at large.